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Diseases The Ear

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The ear can be divided into three parts: outer ear, middle ear and inner ear. The outer ear collects sound, whose pressure is amplified through the middle ear and passed from a medium of air into a medium of fluid. The change from air to fluid occurs because air surrounds the head and is contained in the ear canal and middle ear, but not in the inner ear. The hollow channels of the inner ear are filled with fluid, and lined by a sensory epithelium that is studded with hair cells. The hair cells can convert sound waves into nerve impulses, which then travel to the brainstem. The information is further processed and eventually reaches the thalamus, and from there it is relayed to the portion of the cerebral cortex dedicated to sound. This auditory cortex is located in the temporal lobe.

Outer Ear

The outer ear includes the pinna and the ear canal (also called the auditory canal). The pinna is the folds of cartilage surrounding the ear canal. In some animals with mobile pinnae (like the horse), each pinna can be aimed independently to better receive the sound. For these animals, the pinnae help localize the direction of the sound source. Human beings localize sound within the central nervous system, by comparing loudness from each ear in brain circuits that are connected to both ears.

Earwax (medical name - cerumen) is produced by glands in the skin of the outer portion of the ear canal. It is a mixture of viscous secretions from sebaceous glands and less-viscous ones from modified apocrine sweat glands. Ear wax helps clean any dirt, dust, and particulate matter that may have gathered in the canal. It also plays a role in lubrication, and provides some protection from bacteria, fungus, and insects.

Middle Ear

The middle ear includes the eardrum (also called tympanic membrane) and ossicles. The ossicles are, in order from the eardrum to the inner ear, the hammer, anvil, and stirrup, so named because of the shape of the bones. They are also commonly referred to by the equivalent Latin terms: malleus, incus, and stapes respectively.

  • The malleus articulates with the incus and is attached to the eardrum, from which vibrational energy is passed.
  • The incus is connected to both the other bones.
  • The stapes articulates with the incus and is attached to the membrane of the fenestra ovalis, the elliptical or oval window or opening between the middle ear and the vestibule of the inner ear.

As sound waves vibrate the eardrum, it in turn moves the nearest ossicle, the malleus to which it is attached. The malleus then transmits the vibrations, via the incus, to the stapes, and so ultimately to the membrane of the fenestra ovalis, the opening to the vestibule of the inner ear.

The ossicles give the eardrum mechanical advantage via lever action and a reduction in the area of force distribution; the resulting vibrations would be much smaller if the sound waves were transmitted directly from the outer ear to the oval window. However, the extent of the movements of the ossicles is controlled (and constricted) by certain muscles attached to them (the tensor tympani and the stapedius). It is believed that these muscles can contract to dampen the vibration of the ossicles, in order to protect the inner ear from excessively loud noise and that they give better frequency resolution at higher frequencies by reducing the transmission of low frequencies

Inner Ear

The inner ear is also called labyrinth, which was named by analogy with the mythical maze that imprisoned the Minotaur. It consists of two parts:

  • Cochlea - the organ of hearing.
  • Vestibular System - responsible for balance.
Cochlea

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The cochlea is filled with a watery liquid, which moves in response to the vibrations coming from the middle ear via the oval window. As the fluid moves, thousand of "hair cells" are set in motion, and convert that motion to electrical signals that are communicated via neurotransmitters to many thousands of nerve cells. The nerve impulses travel along the auditory nerve to structures in the brainstem for further processing.

The stapes of the middle ear transmits to the fenestra ovalis (oval window) on the outside of the cochlea, which vibrates the perilymph (fluid) in the scala vestibuli (upper chamber of the cochlea).

This motion of perilymph in turn vibrates the endolymph in the scala media, thus causing movements of the hair bundles of the hair cells, which are acoustic sensor cells that convert vibration into electrical potentials. The hair cells in the organ of Corti are tuned to certain sound frequencies, being responsive to high frequencies near the oval window and to low frequencies near the apex of the cochlea.

The hair cells are arranged in four rows in the Organ of Corti along the entire length of the cochlear coil. Three rows consist of outer hair cells (OHCs) and one row consists of inner hair cells (IHCs). The inner hair cells provide the main neural output of the cochlea. The outer hair cells, instead, mainly receive neural input from the brain , which influences their motility as part of the cochlea's mechanical pre-amplifier. The input to the OHC is from the olivary body via the medial olivocochlear bundle.

For very low frequencies (below 20Hz), the pressure waves propagate along the complete route of the cochlea - up scala vestibuli, around helicotrema and down scala tympani to the round window. Frequencies this low do not activate the organ of Corti and are below the threshold for hearing. Higher frequencies do not propagate to the helicotrema but are transmitted through the endolymph in the cochlea duct to the perilymph in the scala tympani.

A very strong movement of the endolymph due to very loud noise may cause hair cells to die. This is a common cause of partial hearing loss and is the reason why users of firearms or heavy machinery should wear earmuffs or earplugs.

Vestibular System

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The vestibular system consists of three semicircular canals and the vestibule. The three semicircular canals are responsible for detecting rotational movements while the otolithic organs in the vestibule detect linear acceleration (movement in a straight line). There are two otolithic organs: utricle and saccule. The three semicircular canals are called horizontal (or lateral), anterior (or superior) and posterior (or inferior). They are approximately orthogonal to each other. The vestibular system interacts with other systems in the body, such as the visual (eyes) and skeletal (bones and joints) systems, to maintain the body's balance, as discribed below.

Balance

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The semicircular canals are fluid-filled. Movement of fluid signals the brain about the direction and speed of rotation of the head -- for example, whether we are nodding our head up and down or looking from right to left. Each semicircular canal has a bulbed end, or enlarged portion, that contains hair cells. Rotation of the head causes a flow of fluid, which in turn causes displacement of the top portion of the hair cells that are embedded in the jelly-like cupula.

Two other organs that are part of the vestibular system are the utricle and saccule. These are called the otolithic organs and are responsible for detecting linear acceleration. The hair cells of the otolithic organs are blanketed with a jelly-like layer studded with tiny calcium stones called otoconia. When the head is tilted or the body position is changed with respect to gravity, the displacement of the stones causes the hair cells to bend.

As in the cochlea, movement of hair cells can be converted into electrical signals, traveling along nerve cells to other regions. The balance system works with the visual and skeletal systems (the muscles and joints and their sensors) to maintain orientation or balance. For example, visual signals are sent to the brain about the body's position in relation to its surroundings. These signals are processed by the brain, and compared to information from the vestibular and the skeletal systems. An example of interaction between the visual and vestibular systems is called the vestibular-ocular reflex. The nystagmus (an involuntary rhythmic eye movement) that occurs when a person is spun around and then suddenly stops is an example of a vestibular-ocular reflex.

 

Sources:

Wikipedia

National Institute on Deafness and Other Communication Disorders, USA